1. Field
The present invention relates generally to telecommunications, and more specifically to a novel and improved method and apparatus for reducing a peak-to-average power ratio of a reverse link signal.
2. Background
Communication systems that consist of independently modulated channels can result in a high peak-to-average power ratio. Reverse link channels in cdma2000 (1x) and cdma2000 (1x-EV) systems are examples of independently modulated channels. Reverse-link signals are those signals that are sent from a subscriber unit to an infrastructure element in a wireless communication system. A high peak-to-average power ratio results in reduced efficiencies for a subscriber unit""s RF power amplifier (hereinafter power amplifier). For example, a high peak-to-average amplitude waveform places greater demands on the power amplifier of a subscriber unit""s transmitter and can reduce the maximum data rate or maximum range at which a subscriber unit can operate. This is due to several factors, the most important of which is that a waveform with a higher peak-to-average power ratio by definition transmits a larger maximum power for the same average transmit power. Therefore, a power amplifier with a greater linear range is required and these kinds of power amplifiers tend to be more expensive. In addition, a larger maximum power requires more current, which reduces subscriber unit battery life. Thus, a high peak-to-average power ratio results in the reduced battery life of a subscriber unit.
It is desirable to reduce the peak-to-average power ratio of the reverse link signal because a benefit of reducing the peak-to-average power ratio is a longer battery life. Another benefit of a lower peak-to-average power ratio is reduced backoff from a power amplifier compression point, which results in greater average transmit power or greater range. Amplitude and phase non-linearity of power amplifiers, known as AM/AM and AM/PM non-linearity, respectively, results in an inter-modulation distortion and requires operation of amplifiers much below a power amplifier""s compression point. A one dB output compression point of a power amplifier is an output power level at which gain deviates from an output signal gain by one dB. For example, a one dB output compression point is the output power level at which a ten dB increase in input power results in only a nine dB increase in output power.
A reduced peak-to-average power ratio can manifest itself as either greater range (i.e., more average output power) and/or increased battery life for a given level of average power (peak power transmissions use more current).
One simple method to reduce the peak-to-average power ratio is to clip a signal such that the amplitude and consequently the peak-to-average power ratio are limited to some acceptable maximum level. Bandwidth regrowth or out-of-band leakage due to clipping is a critical issue for wireless systems. The amount of out-of-band spill determines filtering requirements and adjacent channel interference.
Clipping in the time domain is convolving a rectangular pulse with a power spectrum of a reverse-link transmit waveform in the frequency domain. Therefore, clipping introduces a significant amount of in-band noise as well as out-of-band spurious emissions and consequently does not meet the spurious emissions requirements outlined in the xe2x80x9cTIA/EIA-98-D Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Stationxe2x80x9d (the IS-98 standard).
Thus, there is a need in the art for a method and apparatus to reduce the peak-to-average power ratio of a reverse link signal while limiting in-band noise as well as out-of-band spurious emissions.
Embodiments disclosed herein address the need to reduce the peak-to-average power ratio of a reverse-link signal. Reducing the peak-to-average power ratio of the reverse-link signal increases coverage range/distance at which a subscriber unit can successfully operate and/or the battery life of a subscriber unit.
In one aspect, an apparatus comprises a peak-to-average reduction module and a power amplifier. The peak-to-average reduction module is configured to generate a reduced peak-to-average signal in response to peaks in a signal that exceed a target threshold value by applying a reduction function to the signal until the peaks are reduced to or below the target threshold value, or the reduction function has been applied a given number of times. The power amplifier is configured to amplify the reduced peak-to-average signal and to generate an amplified signal. In one aspect, the apparatus is a subscriber unit. In one aspect, the signal is a CDMA signal. In one aspect, the reduction function is an inverted raised cosine function.
In one aspect, the apparatus comprises a reverse link modulator configured to provide channel data, a channel gain calculation module coupled to the reverse link modulator and configured to compute reverse link channel gain, an averaging module that is coupled to the channel gain calculation module and configured to calculate average power for a combination of independent channels on the reverse link, a threshold module coupled to the averaging module and configured to generate envelope thresholds using a target peak-to-average ratio and the average power, a window weight module coupled to the threshold module and configured to align a peak reduction window with a waveform peak and generates window weights to be used for multiplying with filtered samples, a filter coupled to the reverse link modulator and configured to filter an n-chip (where n is an integer) sample of channel data to generate filtered samples, and a multiply module coupled to the filter and configured to multiply the filtered samples at n times a chip rate with an aligned peak window using the window weights from the window weight module to create modified samples with a reduced peak-to-average power ratio.
In one aspect, a method of processing a signal comprises comparing a peak of the signal to a target threshold value, applying a reduction function to the peak if the peak is above the target threshold value until the reduction function is at or below the target threshold value to generate a reduced peak-to-average signal, and terminating application of the reduction function once the reduction function has been applied a given number of times.